One well known type of pipe joint, known as the "Zap Lok" joint is described in Canadian Patents Nos. 869429 and 1103340 (or U.S. Pat. Nos. 4,064,619 and 4,120,083). This is not a welded pipe joint, but is a pin and bell joint making use of a mechanical interference fit. While the "Zap Lok" joint, which is especially suitable for externally coated pipes, is mechanically strong and reliable, it may not be used with pipes that have to carry corrosive or hazardous fluids since in many such applications only welded joints are allowed.
Another known type of pipe joint, known as the "Thru Kote" joint is described in U.S. Pat. No. 3,508,766. This joint is designed particularly for internally lined pipes. The pipes have belled ends which are welded together, a lined insert being accommodated within the belled ends of the pipes to provide continuity of the lining protection through the joint. However, a serious shortcoming of this joint is that leaks may develop in the annular regions between the ends of the insert and the belled ends of the pipes, and in assembling the joint where centering is difficult there is nothing to prevent movement of the insert prior to the actual welding step; in other words, the sealing material of the joint can be disturbed during assembly.